First of all, cracking shots Starcrazzy. That is a reet good collection of galaxies.
For your question -I have pondered this recently and this is why it is foremost on my mind.
How long can you go with an unmodified DSLR?
Well for me with a 1000D I reckon anything under 10 minutes is ok as long as the ambient temperature is below 17'c or around that and dropping. I have severe sensor noise above that with long exposures and the darks have too.
You have a better camera than me (everybody has!) but I think unless you cooled your DSLR or allowed lots of time between lights, or shot in Antarctic conditions, you would be running into sensor noise and and hot pixels. The bane of DSLR's but they are cheaper so woo hoo !! and your pictures are very good anyway
